1. Field of the Invention
The invention generally relates to bismuth-containing oxide films. In particular, the present invention relates to a process for manufacturing bismuth-containing oxide thin films by Atomic Layer Deposition (ALD).
2. Description of the Related Art
Bismuth is a component of several technologically important binary and multicomponent oxide thin film materials, particularly the ferroelectric oxides Bi4Ti3O12, (Bi,La)4Ti3O12, and SrBi2Ta2O9 and, also, the superconducting oxide Bi2Sr2CaCu2O8+x. Ferroelectric bismuth oxide films have great potential use in ferroelectric memory cells. In such uses, the films are likely to be formed with capacitors in the form of 3-D structures, which means that good conformality of the ferroelectric film is desired to ensure proper function of the memory cell.
Atomic Layer Deposition, in the following also abbreviated “ALD”, is a deposition method in which a product, such as a thin film, is deposited on a substrate from precursors in the vapor phase. ALD is based on sequential self-saturated surface reactions. The characteristics of an ALD process are described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811 which are incorporated herein. The reactors designed for ALD benefit from the usage of inert carrier and purging gases, which makes the system faster.
According to the principles of the ALD, the source chemicals (or “precursors”) are separated from each other by inert gases (i.e., by purging) which substantially prevents gas-phase reactions between gaseous reactants, thereby facilitating the film growth by the above-mentioned self-saturated surface reactions. Advantageously, ALD requires neither strict temperature control of the substrates nor precise dosage control of source chemicals. Surplus chemicals and reaction by-products are removed from the reaction chamber before the next reactive chemical pulse is introduced into the chamber. Undesired gaseous molecules are effectively expelled from the reaction chamber by keeping the gas flow speeds high with the help of an inert purging gas. The purging gas pushes the extra molecules towards the vacuum pump used for maintaining a suitable pressure in the reaction chamber. Advantageously, ALD provides an excellent and automatic self-control for the film growth as well as outstanding conformality.
Based on its general properties, ALD is a potentially attractive alternative for deposition of bismuth-containing oxide thin films. However, prior art attempts at the deposition of bismuth containing oxides by ALD had a number of shortcomings. The main problem has been finding appropriate bismuth-oxygen source combinations for depositing bismuth oxide. In an earlier article[M. Schuisky et al., Chem. Vap. Deposition 6 (2000) 139], triphenyl bismuth, a commonly used CVD precursor, was examined for depositing Bi4Ti3O12. No binary Bi2O3 could be deposited, but bismuth could be incorporated into Bi—Ti—O mixture films. However, the levels were too low for forming the desired Bi4Ti3O12 phase. Application of BiCl3 and water, in turn, has resulted in a BiOCl phase (M. Schuisky et al.). Trimethyl bismuth is another common bismuth (Bi) CVD precursor, but it is reported to be explosive. Quite recently, Bi(OCMe2CH2OMe)3 was introduced as a new Bi CVD precursor [P. A. Williams, Chem. Vap. Deposition 7 (2001) 205]. The present inventors tested this compound in ALD and found that it decomposes too easily when heated inside the reactor. Other tested compounds include Bi(CH2SiMe3)3, for which no film growth has been observed with water as an oxygen source, and Bi(thd)3, which appears to be quite unstable based on TG measurements.
In their paper, Roy Gordon et al. (Chem. Mater. 13 (2001) 2463) reported the use of La silylamide for deposition of the corresponding oxide and a mixture of said compound with silicon oxide. The article further contains—it would appear —an exhaustive list of elements forming volatile silylamides. The article is, however, silent about the use of bismuth-containing silylamides.
As the above surveys show, there are no satisfactory processes known for producing bismuth-containing ferroelectric films by the Atomic Layer Deposition technique.